The present invention relates to a laser diode which has excellent noise characteristics without astigmatism.
In so-called index guided type elements where laser light distribution (transverse mode) of a laser diode is confined by the refractive index step between a stripe inside portion and an outer edge portion, the lasing spectrum (longitudinal mode) becomes singular. When such an element is used for a compact disk, a feedback induced noise caused by reflected light from the disk surface may be produced. On the other hand, in an element with a smaller index step, although the longitudinal mode becomes a multi mode oscillation and the feedback induced noise is not produced, the position of the beam west is different in the horizontal direction to an active layer than in the vertical direction producing a so-called astigmatism and making focusing of the laser beam difficult. Consequently, an element in which the longitudinal mode is multi mode and in which there is no astigmatism is desirable. The above object can be attained if the refractive index is made smaller inside the element and the index step of the stripe is made larger at a portion near one edge by varying the stripe structure in the optical axis direction of the laser diode. An element in which the index step is made larger at a portion near one edge of the element has been reported in Shimada et al, "characteristics of rib guide type GaAlAs laser added edge mode filter", No. 31 Applied Physics lecture (Mar. 29-Apr. 2, 1982) (patent publication No. 5273/1979). However, this element has a structure different from a conventional structure in which semiconductor layers are grown on a channeled substrate. Consequently, control of the channel depth at a portion near the element edge is difficult during the manufacturing process. Furthermore, crystal growth must be performed twice, and the structure cannot be repeatedly produced with ease.